The present invention relates to a method of fabricating electrical conductors for an integrated circuit (IC) having improved electrical properties. The invention effectively reduces the dielectric constant of insulating material between selected IC conductors on the same level (intralevel) or between conductors on superposed levels (interlevel) in VLSI or ULSI circuits, hence dramatically reducing the coupling capacitance between conductor levels. Realization of this method and structure utilizes an enclosed near-unity dielectric constant gas or liquid material, the composition and pressure of which can be selected.
There is a need to replace the inorganic/organic insulating material used to isolate metal conductors in ultra large scale integrated circuits (ULSI) in order to reduce the signal RC delay, resulting in a faster (higher frequency) circuit performance. The term xe2x80x98RC delayxe2x80x99 stands for xe2x80x98resistance-capacitance delayxe2x80x99 and is a function of the type of metal conductor used (resistance component) and the type of insulating material used to isolate the metal conductors (capacitive component). The lower the xe2x80x98dielectric constantxe2x80x99, k, of the insulating material, the lower the capacitive component. An ideal gas has a dielectric constant equal to 1.0, whereas most inorganic/organic insulating materials currently used in the semiconductor industry have a dielectric constant of 2.5-3.0 (organic polymers) to 4.3 (inorganic silicon dioxide), or even higher (silicon nitride). Historically, silicon dioxide has been used as the insulating material for on-chip ULSI interconnect purposes. However, with the need to make integrated circuits smaller and faster (resulting in faster computers, etc.), there has been a concerted effort by the semiconductor industry to find a replacement for silicon dioxide. A number of xe2x80x9corganicxe2x80x9d insulating materials having a lower dielectric constant are being considered, however, most of these materials have reliability problems.
The RC delay associated with interconnect is rapidly becoming the limiting factor in realizing high speed integrated circuits with design rules below 0.25 microns. See National Technology Roadmap for Semiconductors: Technology Needs. Published by Semiconductor Industry Assoc., pp 99-110, 1997.
As packing density increases, the cross-sectional area of interconnect lines decreases causing the resistance to length ratio to dramatically increase. The adoption of copper as the conductor of choice can improve the resistance component by almost a factor of two over that of aluminum (from 3.0 to 1.7 micro-ohm-cm resistivity). However, a dramatic reduction in the dielectric constant of the intermetal dielectric material over that of silicon dioxide (k=4.1) is also needed to address the capacitive component for future high speed circuitry.
A reduction in interconnect line capacitance (resulting from a reduction of dielectric constant) can reduce signal propagation delay, reduce power consumption at high frequencies and reduce cross coupling between conductors (cross-talk and noise). Another significant benefit of reducing RC delay is to reduce process complexity (i.e., 12 levels of metal using Alxe2x80x94SiO2 versus 6 levels of metal using Cu-low k at 0.13 microns design rules), and hence improves reliability and yield while reducing cost. A reduction of wire capacitance can also provide an increased degree of design freedom; the designer can use the reduction in capacitance to either improve speed or reduce power. See G. A. Sai-Halasz, Proc IEEE, vol 83, no. 1, p 20, 1995. Currently, there is a wide variety of organic and inorganic materials bring investigated as potential candidates for low-k intermetal dielectrics. See D. S. Armbrust, D. Kumar, Short Course on Dielectrics for ULSI Multilevel Interconnection Visuals Booklet, DUMIC, Santa Clara, Calif., Feb. 10, 1999. Also, procedures have been proposed to conduct comparative evaluations of these candidates in order to find the optimal material. See T. E. Wade, xe2x80x9cOptimum Dielectric Selection Using a Weighted Evaluation Factorxe2x80x9d, DUMIC, pp 211-218, 1995 and Semiconductor International, pp 99-106, vol 38, no. 8, August, 1995. Many of these candidates exhibit severe reliability, manufacturability and/or process integration problems, especially the organic candidates.
Properties required for an acceptable intermetal dielectric material for use in ULSI interconnects include a) low dielectric constant (ideally k=1.0), b) high breakdown field strength ( greater than 2 MV/cm), c) low bulk leakage (resistivity greater than 1015 ohms/cm), d) low surface conductance (surface resistivity greater than 1015 ohms), e) low stress (compressive or weak tensile greater than 30 Mpa), f) mechanical/chemical/thermal stability, g) no moisture absorption and/or permeability to moisture, h) process compatible (CMP/dual damascene/etc.), i) good thermal properties (high thermal conductivity, low TCE, stable), j) compatibility with environmental, health and safety requirements, etc. The National Semiconductor Roadmap for Semiconductors calls for dielectrics with k=2.5-3.0 for 0.18 micron devices and 2.0-2.5 at 0.15 micron devices. If a reliable unity-k dielectric system could be realized using conventional technological processes, a quantum step towards meeting Roadmap goals could be achieved.
The use of gas dielectrics offers many benefits, including: 1) optimal electrical properties (unity dielectric constant (k=1) for reduced RC delay/cross-talk/power consumption, high breakdown strength, low leakage, high volume and surface resistivity, no polarization effects, low ionic/contamination/migration effects, low mobile ion/charge trapping effects), 2) optimal mechanical properties (no shrinkage, no stress due to thermal intrinsic effects, no problems with adhesion, no defect density issues like pinhole density/particulates/cracks/seams/etch pits/etc., no gap fill problems, no planarization problems), 3) optimal chemical properties (resistant to corrosion, leaching and precipitation, no EHS issues), and 4) optimal design/processing characteristics (scalability, reduced complexity/cost/improved yield, no barriers needed, reduced overall cost-of-ownership, commercially available sub-processes).
To date, liquid dielectric materials have not been used as insulators in the fabrication of integrated circuits. However, current research may well result in liquid (or semi-liquid) dielectric materials having dielectric properties similar to those mentioned above for gases but with superior thermal conduction properties.
The most significant benefits of utilizing gas (and possibly liquid) dielectrics are reduced cost, improved yield and higher speed circuitry. Also, in general the greatest benefit for unity-k dielectrics is where lines are at their minimum pitch.
A method of providing an electrically insulating medium in an enclosed envelope which contains multilevel metal conductors of an integrated circuit is disclosed. The method includes the step of providing a base substrate. The base substrate is formed of insulating material. Next, a plurality of discrete multilevel metal conductors are formed on and above the base substrate, and then a plurality of discrete support means are formed to extend upwardly from the base substrate to one or more conductor levels or between conductor levels. A selectively removable material is then deposited on the base substrate and around the support means and the metal conductor.
A dome layer of insulating material is then provided overlying the support means, the conductor levels, and the removable material. Once the dome layer is provided, access opening means are formed in the dome layer to communicate with the removable material. The removable material is then removed through the access opening means. The removable material is removed without interrupting the base substrate, the dome layer, the support means, and the metal conductors. As such, the envelope is defined between the base substrate and the dome layer and around the support means and the metal conductors such that the envelope is filled with a low dielectric constant material. Finally, the access opening means are sealed with conducting or insulating material as desired.
Ultimately, this invention provides a method and structure for providing a multilevel conductor system for ULSI circuitry having the following unique features:
1. Multilevel conductors within a xe2x80x98domed envelope regionxe2x80x99 which are completely surrounded (top, bottom, and each side) by a near unity-k dielectric material.
2. All conductors within the xe2x80x98domed envelope regionxe2x80x99 are held in place by a plurality of stanchions formed by inorganic insulating materials (except those penetrating the large inorganic support blocks under bonding pads).
3. The dielectric material contained within the xe2x80x98domed envelope regionxe2x80x99 could be either:
a) any non-intrusive near-unity-k gas, at a pressure that is below or above atmospheric pressure (i.e., for purposes of heat extraction, gases at pressures above atmospheric pressure are desirable).
b) any non-intrusive near-unity-k liquid which possesses good electrical/mechanical/reliability properties and excellent thermal conductivity capability.
4. Large metallic thermal columns that run through the xe2x80x98domed envelope regionxe2x80x99 for the purpose of extracting heat from this region and channeling it to the upper surface of the integrated circuit structure for removal.
5. Large inorganic support blocks placed within the xe2x80x98domed envelope regionxe2x80x99 directly beneath bonding pads (which are located on the upper surface) for the purpose of providing mechanical integrity during the external wire bonding application.
6. A method of efficiently extracting the sacrificial (polymer) material from the xe2x80x98domed envelope regionxe2x80x99 by using a metallic xe2x80x98vapor blockxe2x80x99. This xe2x80x98vapor blockxe2x80x99, after used to extract the sacrificial material, is altered to become a thermal metallic column in the end product (see item 4 above).
7. Multilevel conductors are fabricated in the xe2x80x98domexe2x80x99 layer using state-of-the-art duel-damascene processing techniques. The inorganic xe2x80x98domexe2x80x99 layer has a higher dielectric constant, however, for upper layer conductors the conductor size is somewhat large and the spacing between conductors can be increased considerably so as to minimize the capacitive coupling between these conductors.
Moreover, conventional low-k/copper dual damascene processes can be utilized to realize this multilevel interconnect structure having an effective (gas) dielectric constant of near unity, resulting in a substantial interconnect capacitance reduction for tightly spaced metal lines. Also, no organic dielectric materials (and their associated reliability problems) are included in the final multilayer structure.